Changing the internal energy by
heating and by work

Working

When a system such as
an ideal gas does work its internal energy is reduced. If compressed CO2
gas is released through a nozzle it will do work in expanding against air
pressure and so its internal energy falls and the gas will cool and solidify.

If the system has
work done on it its internal energy is increased. For example gas being
compressed in a piston has its internal energy increased and so its temperature
increases.

When a system does work its internal
energy is reduced.

When
a system has work done on it its internal energy is increased.

Heat transfer

If heat energy is
transferred from the surroundings to a system then the system will increase its
internal energy and the temperature of the system will increase. If the system
expands when the heat is transferred (ie. the system does work) then the
internal energy of the system will decrease by an amount equal to the work done.

Q is positive if heat
is transferred to the system

Q is negative if heat
is lost from the system

W is positive if work
is done on the system

W is negative if work
is done by the system

The internal energy
is stored as PE in the interatomic bonds, continuously being stretched and
compressed. It depends on the forces between the atoms and their separation.
Where the intermolecular forces are weak the internal energy is nearly all KE
and thus we see a link between kinetic theory, internal energy and temperature.

In an ideal gas
only molecular KE is present. No work is done increasing the molecular
separation (an ideal gas is assumed to have no forces of attraction between the
molecules). Thus the internal energy of an ideal gas is independent of the
volume of the gas and it will obey Boyle’s Law exactly.